docs: def argument_kind(args):
      # type: (List[Argument]) -> Optional[str]
      """Return the kind of an argument, based on one or more descriptions of the argument.
  
      Return None if every item does not have the same kind.
      """
      kinds = set(arg.kind for arg in args)
      if len(kinds) != 1:
          return None
      return kinds.pop()